1. Field of the Invention
This invention relates generally to fluid separation, and more particularly to a new and improved method and apparatus for separating dispersed liquid from a continuous fluid.
2. Description of the Prior Art
Many industrial processes involve contacting operations where droplets or particles of liquid are dispersed in a gas phase or in another liquid phase in the form of a mist, fog, fume, droplets or similar dispersion. The particles may vary widely in size from several hundredths of a micron to several hundred micron and their separation is often necessary to recover or purify either or both the liquid and gas components.
Some of these processes generate gas streams containing valuable entrained organic liquid compounds such as alcohols and petrochemicals, or they create hazardous workplaces with liquid particles of acids, ammonia, oils, etc. entrained in the air. Sources of oil mist, for instance, may be derived from compressed air and machine shops. Another process involves erosion of steam turbine impellers by water particles contained in steam. To prevent the particles from forming, the steam is superheated but at a significant waste of energy. There are also many industrial examples in which a dispersed liquid must be removed from a continuous liquid. Oil spills in streams, lakes, etc. and oily contaminants in drinking water are good examples.
Various devices for the physical separation of liquid particles entrained in a gas or another liquid utilize differences in physical properties such as particle size, density, velocity, wettability, and electric and magnetic characteristics. One device generates a centrifugal force to remove the liquid particles by increasing their inertial velocity in a rotating device, especially of large or heavy particles. However, this is usually not suitable for the separation of liquid particles in a gas because they may break up and vaporize upon contact with the rotating device.
Electric or magnetic force fields are also applied to liquid particles to remove them from a gas or another liquid. Electrostatic precipitation for example, effectively separates submicron-size liquid particles from a gas stream, but because of the large size of equipment required the capital investment is generally high and it is not applicable to combustible mixtures. Other electrostatic methods of separation, such as electrostatic adsorption, electrostatic filtration, electrostatic repulsion, electrostatic double-layering, elestrophoresis etc. have drawbacks similar to the above mentioned.
Another form of separator is the scrubber. It utilizes a spray chamber, spray cyclone or packed tower, in which a stream of water impinges the particles on solid surfaces. These forms can be effective for separating liquid particles insoluble in water, but it generates waste water which must also be treated before it is discharged into the environment.
Particles several microns and larger can be separated by gravity settling, direct interception and inertial deposition while smaller sub-micron particles are collected by Brownian diffusional deposition inside a fiber bed. However, the size of this type of separator is relatively large and usually requires a long particle residence time and/or high gas pressure.
Another form of separator employs a screen or filter such as a sieve, septum, or membrane in which impingement surfaces retain the liquid particles and allow the gas to pass. The particles, with their inertial forces due to the gas, strike and adhere to solid surfaces. This form is not very effective because the particles accumulated near the solid surface can prevent further flow of the gas through the screen or filter and allow the particles to reentrain if the gas velocity is high. The efficiency is also limited because the gas back-mixes inside the separator lowering the net separation rate of the particles. Furthermore, the size of equipment is relatively large for dilute mixtures.
Conventional impingement separators exist in a wide variety of forms such as disclosed in Perry's Chemical Engineers' Handbook. Sixth Edition, McGraw-Hill, Inc. .COPYRGT.01969, pp. 18-70 to 18-84. The simplest form consists of a nozzle and target of solids or a container of inert packing material through which the liquid particles and gas slowly pass. The particles impinge and collect on the solids or packing material and are removed in a route separate from the gas stream. Examples of other forms are jet impactors, wave plate separators, staggered channels, vane-type mist extractors, zigzag plate separators, staggered vane separators, baffle separators, and wire-mesh de-misters.
Impingement separators are best suited for large entrained liquid particles moving at high velocities. The efficiencies are low because there is back-mixing of flowing gas and reentrainment of the particles. Efficiencies in the range of 20% to 70% are typical in industrial operations.
For the impingement separation of fine mists or fog with liquid particles less than one micron in diameter, a cylindrical bed of randomly packed fibrous materials such as glass or polypropylene lining the inside of a vertically disposed cylindrical separator is used. The liquid-gas mixture is introduced through a side-wall, passes through the fibrous bed under pressure. The liquid particles impinge on the fibrous material and coalesce to form a continuous film which drains by gravity out the bottom of the separator while the gas exits through the top. Liquid particles one micron and larger move in the gas according to Stokes' law and separate readily from the gas upon impingement. Smaller particles move more slowly through the material in a random motion close to Brownian movement. On the average, all of the particles travel across a distance equal to half the inside diameter of the cylinder. For. mixtures having a wide range of liquid particle sizes, a larger filter area, longer residence time, and higher gas pressure are required to maintain the same separation efficiency and throughput capacity. Several impingement beds placed in series may be needed to improve efficiency, or placed in parallel to increase throughput capacity. The trade-off is more capital investment and higher operational costs.
Efforts to improve the separation efficiency of the cylindrical impingement separator have been tried by placing a fan inside the unit or rotating the cylindrical impingement filter inside in order to generate more dynamic conditions and greater centrifugal force in the gas as it flows through the fibrous bed. The fan and the rotational motion break up and vaporizes the liquid particles creating a large amount of liquid vapors in the treated gas stream. The separation efficiency of the unvaporized liquid particles may be higher but separation of the total liquid component in the gas is lower.
Some liquid particle-gas separators use both impingement and centrifugal force. One separator of this type has a number of target plates arranged in a circular fashion inside a cylindrical tank. The wet gas flows in a vortex through the spaces between the plates to produce a greater impingement effect. The separator is frequently used for treating wet steam. The steam enters the tank tangentially near the top and leaves through a central outlet, while liquid is drained from the bottom of the separator. Another type has a rotating surface which produces a vortex of the liquid particle-gas mixture before it impinges on target plates on the inside wall of a cylindrical tank. The mixture enters at the top, the gas leaves at the bottom, and the liquid particles are drained from a separate outlet near the bottom. Still another type separator has a curved baffle which creates a vortex of the incoming liquid particle-gas mixture at the top of a cylindrical tank and directs it toward target plates arranged on the inside wall for impingement. Liquid particles are drained from the bottom of the separator. Each of these separators is only effective for separating large liquid particles and not for a fine liquid mist or fog because the fine particles move at very low velocities with little or no centrifugal force. While the separation efficiency of liquid particles larger than one micron may be improved by combining centrifugal force with impingement, the investment in capital is comparatively large.
From all of the above, it is apparent that the turbulence and back-mixing of fluid inside conventional impingement separators are pervasive problems because of reentrainment of particles and low separation efficiency. Low flow rates or large residence time of gas reduces turbulence and back-mixing, but at the expense of throughput capacity. Larger equipment may handle larger flow rates, but the travel distance for the particles is increased making the equipment less efficient. Parallel or series arrangements of an equipment increases capital and operational costs. Higher operational pressures increase the transfer rate of very small particles that move by the Brownian motion, but at the expense of higher capital costs. Separation equipment designed for separation of large particles is not effective for separation of small particles, such as in a fog or fume. Impingement filters may be designed for separating particles of a wide size range, but only for very slow flow rates or very high residence time required for small particles. Centrifugal force is beneficial for impingement of large particles, but the rotating equipment used to produce the force can break up and vaporize liquid particles in contact.
Most separation principles for liquid-gas mixtures are equally applicable to separation of liquid-liquid mixtures in which one or more liquids are dispersed in another liquid except that the design and the operation of conventional separation equipment varies greatly from that for gas-liquid mixtures. Early conventional separation technology is based on coalescence by impingement followed by simple phase separation. It is effective only for separation of larger droplets, and requires a large residence time since liquid turbulence will redisperse once coalesced particle. Furthermore, the equipment must provide a large surface area for a greater rate of impingement and a shorter distance for short particle transfer times. The capital and operational costs of this technology are relatively low.
Flotation and foam separation using coagulating and flocculating agents or foaming agents are very effective and could achieve complete separation of liquid-liquid mixtures containing a very fine dispersed liquid if the turbulence and back-mixing of the liquid mixture were prevented.
For the separation of sub-micron size dispersed liquids, industry widely and very effectively uses centrifugation and membrane separation. However, at high-speeds the expenditure is relatively high. Membranes used for micro-filtration are usually made specifically for a given system for best results. In membrane filtration, only the continuous fluid is allowed to pass while the dispersed liquid particles blocked by the membrane accumulate in the fluid mixture being treated. Consequently,. pressurization of the fluid being treated and/or vacuuming of the treated liquid are often required with all attendant increases in costs of equipment and maintenance.
Electric or magnetic force fields can also be used to separate liquid-liquid mixtures, although the separation rates are usually very slow due to higher viscosities of liquid-liquid mixtures. In order to increase the separation rate and efficiency, the particle transfer distance must be shortened and the contacting surface must be increased. Furthermore, liquid turbulence inside the equipment must be prevented completely for the separation to take place. Therefore, solid surfaces and high surface-potential fields are often used for generating electrostatic attraction or repulsion forces for separation, instead of using low external fields in the liquid. Capital and operational costs of electric and magnetic separators are relatively high.